最初,对溶瘤作用的研究通常基于野生型或天然存在的病毒,例如西尼罗河病毒、狂犬病病毒、黄热病毒、肝炎病毒等[4]。2000 年前后,科学家开始通过基因工程对野生型病毒进行一系列改造。修饰的溶瘤病毒可以配备所需的外源基因,这些基因可以通过不同的机制发挥抗肿瘤作用[5]。从而大大地促进了溶瘤病毒的研究进程以及应用范围。2015年,美国 FDA 批准 Imlygic(Talimogene laherparepvec; T-VEC)用于黑色素瘤病变治疗,该药物也是 FDA 批准的首个溶瘤病毒治疗药物[6]。
肿瘤抗原在注射部位与抗原呈递细胞 (APCs)相遇,被抗原呈递细胞(如树突状细胞,DCs)摄取。载有抗原的 APCs 通过淋巴管输送到引流淋巴结,这是 T 细胞启动(T cell priming)的主要部位。在淋巴结中,成熟的树突状细胞将 MHC 分子上的肿瘤衍生肽(tumour-derived peptides)呈递给初始和记忆 T 细胞;共刺激“第二信号”促进肿瘤特异性 T 细胞反应的产生;树突状细胞释放的 IL-12 和 I 型干扰素 (IFNs)促进共刺激作用;这些相互作用共同促进活化的肿瘤特异性 T 细胞群的产生和扩增。活化的 T 细胞运输到肿瘤部位,在遇到它们的同源抗原(cognate antigens)时,它们可以通过细胞毒性和产生效应细胞因子(如 IFNγ 和 TNF)来杀死肿瘤细胞。反过来,裂解的肿瘤细胞释放肿瘤抗原,这些抗原可以再次被 APC 捕获、加工和呈递以诱导多克隆 T 细胞反应,从而增加抗肿瘤免疫反应的抗原广度并导致表位扩散过程[10]。
图 5:癌症疫苗的作用原理 [10]
2010 年 4 月,sipuleucel-T (Provenge; Dendreon)被美国 FDA 批准用于治疗无症状或症状轻微的转移性去势抵抗性前列腺癌(mCRPC)。这是第一种获得 FDA 批准的治疗性癌症疫苗[11]。
人体的免疫功能受到激发时会活化,但不会过度活化,因为免检检查点分子的主要功能之一就是类似车的刹车系统,在免疫系统活化时能够及时“刹车”,使免疫系统的活化保持在正常的范围之内,以防止活化过度而产生自身免疫反应。肿瘤细胞会表达一些物质,来激活免疫检查点,以抑制 T 细胞的免疫功能,肿瘤细胞就能逃脱监视、存活下来。
表 1:已获 FDA 批准的免疫检查点抑制剂 [23] 2011 年,Ipilimumab 被 FDA 批准用于治疗不可切除或转移性黑色素瘤,成为第一个获得 FDA 批准的 ICI[24]。Ipilimumab 的靶点是 CTLA-4,其作用机制如下:
T 细胞活化需要两个信号的刺激。一种是 TCR-MHC 信号转导,另一种是 CD28-CD80/86(B7-1/2) 共刺激。成功共刺激后,初始 T 细胞活化并得以执行其功能。激活后,T 细胞表达 CTLA-4,CTLA-4 与 CD80/86 分子结合,从而使 T 细胞失活。应用单克隆抗体阻断 CTLA-4 可以减轻其对 T 细胞的抑制作用,重新激活 T 细胞增殖并分化为细胞毒性 T 淋巴细胞 (CTL),从而发挥抗肿瘤免疫作用[25]。
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